lemmy/docs/src/administration_configuration.md

The configuration is based on the file defaults.hjson. This file also contains documentation for all the available options. To override the defaults, you can copy the options you want to change into your local config.hjson file.

Additionally, you can override any config files with environment variables. These have the same name as the config options, and are prefixed with LEMMY_. For example, you can override the database.password with LEMMY__DATABASE__POOL_SIZE=10.

An additional option LEMMY_DATABASE_URL is available, which can be used with a PostgreSQL connection string like postgres://lemmy:password@lemmy_db:5432/lemmy, passing all connection details at once.